This invention is concerned with diodes, and more particularly diodes exhibiting low on-state forward voltage and reduced parasitic resistance.
A diode is an electronic component that restricts the direction of movement of charge carriers. The diode essentially allows an electric current to flow in one direction, but substantially blocks current flow in the opposite direction.
Diode rectifiers are one of the most widely used devices in low voltage switching, power supplies, power converters and related applications. For efficient operation, it is desirable for such diodes to operate with low on-state voltage (a forward voltage drop Vf of 0.1-0.4V or lower), low reverse leakage current, a voltage blocking capability of 20-30V, and high switching speed. These features are important to achieve high conversion efficiency, which is the final goal of any rectifier for low voltage applications.
The most common diodes are based on semiconductor pn-junctions, typically using silicon (Si), with impurity elements introduced to modify, in a controlled manner, the diode's operating characteristics. Diodes can also be formed from other semiconductor materials, such as gallium arsenide (GaAs) and silicon carbide (SiC). In a pn diode, conventional current can flow from the p-type side (the anode) to the n-type side (the cathode), but not in the opposite direction.
A semiconductor diode's current-voltage, or I-V, characteristic curve is attributable to the depletion layer or depletion zone that exists at the pn junction between the differing semiconductor layers. When a pn junction is first created, conduction band (mobile) electrons from the n-doped region diffuse into the p-doped region, where there is a large population of holes (locations for electrons where no electron is present) with which the electrons can “recombine”. When a mobile electron recombines with a hole, the hole vanishes and the electron is no longer mobile, i.e., two charge carriers are eliminated. The region around the p-n junction becomes depleted of charge carriers and thus behaves as an insulator.
The width of the depletion zone, however, cannot grow without limit. For each electron-hole pair that recombines, a positively charged dopant ion is left behind in the n-doped region and a negatively charged dopant ion is left behind in the p-doped region. As recombination proceeds and more ions are created, an increasing electric field develops through the depletion zone, which acts to slow and then eventually stop recombination. At this point, there is a ‘built-in’ potential across the depletion zone.
If an external voltage is placed across the diode with the same polarity as the built-in potential, the depletion zone continues to act as an insulator, preventing any significant electric current. This is the reverse bias phenomenon. If the polarity of the external voltage opposes the built-in potential, however, recombination can once again proceed, resulting in substantial electric current through the p-n junction. For silicon diodes, the built-in potential is approximately 0.6 V. Thus, if an external current is passed through the diode, about 0.6 V will be developed across the diode, causing the p-doped region to be positive with respect to the n-doped region. The diode is said to be ‘turned on’, as it has a forward bias.
A diode's I-V characteristic can be approximated by two regions of operation. Below a certain difference is potential between the two leads attached to the diode, the depletion layer has significant width, and the diode can be thought of as an open (non-conductive) circuit. As the potential difference is increased, at some point the diode will become conductive and allow charges to flow. The diode can then be considered as a circuit element with zero (or at least very low) resistance. In a normal silicon diode at rated currents, the voltage drop across a conducting diode is approximately 0.6 to 0.7 volts.
In the reverse bias region for a normal p-n rectifier diode, the current through the device is very low (in the μA range) for all reverse voltages up to a point called the peak inverse voltage (PIV). Beyond this point, a process called reverse breakdown occurs, which causes the device to be damaged, accompanied by a large increase in current.
One disadvantage of a junction diode is that, during forward conduction, the power loss in the diode can become excessive for large current flow. Another type of diode, the Schottky barrier diode, utilizes a rectifying metal-to-semiconductor barrier instead of a pn junction. The junction between the metal and the semiconductor establishes a barrier region that, when properly fabricated, will minimize charge storage effects and improve the switching performance of the diode by shortening its turn-off time. [L. P. Hunter, Physics of Semiconductor Materials, Devices, and Circuits, Semiconductor Devices, Page 1-10 (1970)].
Common Schottky diodes have a lower forward voltage drop than pn-junction diodes and are thus more desirable in applications where energy losses in the diode can have a significant negative impact on the performance of the system, e.g., where diodes are used as output rectifiers in a switching power supply. For such applications, it is highly desirable to provide a rectifier with a very low forward voltage drop (0.1-0.4V), reduced reverse leakage current, low voltage blocking capability (20-30V), and high switching speed. These features are important to achieve high conversion efficiency, which is the final goal of any rectifier that is to be used for low voltage applications.
Schottky diodes can be used as low loss rectifiers, although their reverse leakage current is generally much higher than other rectifier designs. Schottky diodes are majority carrier devices; as such, they do not suffer from minority carrier storage problems that slow down most normal diodes. They also tend to have much lower junction capacitance than pn diodes, which contributes to their high switching speed.
One way to reduce the on-state voltage below 0.5V in a conventional Schottky diode is to reduce the diode's surface barrier potential. Reducing the barrier potential, however, results in a tradeoff of increased reverse leakage current. In addition, the reduced barrier can degrade high temperature operation and result in soft breakdown characteristics under reverse bias operation.
In addition, for Schottky diodes that are made of GaAs; one disadvantage of this material is that the Fermi level (or surface potential) is fixed or pinned at approximately 0.7 volts. (Si Schottky diodes also have this limitation to a certain extent.) As a result, the on-state forward voltage (Vf) is fixed. Regardless of the type of metal used to contact the semiconductor, the surface potential in such a diode cannot be lowered to lower Vf.
One solution to this limitation with GaAs is the gallium nitride (GaN) material system. GaN has a 3.4 eV wide direct bandgap, high electron velocity (2×107 cm/s), high breakdown fields (2×106 V/cm) and the availability of heterostructures. GaN based low voltage diodes can achieve reduced forward voltage drops in comparison with conventional Schottky diode rectifiers (See, e.g., Parikh, et al., Gallium Nitride Based Diodes with Low Forward voltage and Low Reverse Current Operation, U.S. patent application Ser. No. 10/445,130, filed May 20, 2003, which is commonly assigned and the specification of which is incorporated herein by reference as if described in its entirety).
GaN low voltage diodes, however, can be typically fabricated on a SiC or GaN substrate. For a vertical diode device, the substrate is in the conductive path and contributes to the voltage drops. With typical substrate resistivity values of around 20-30 mohm-cm for SiC/GaN substrates, a 200 μm thick substrate will add 40-60 mV of voltage drop at an operating current density of 100 A/cm2. This additional voltage drop is unacceptable, since the target for total voltage drop at operating current is <200 mV. Furthermore, for the most commonly used SiC substrate (GaN substrates are expensive and small in diameter), an additional barrier is encountered at the GaN epi-SiC substrate interface. While there are techniques used to mitigate this barrier, they add extra complexity and may also contribute to increased resistance.
Consequently, a need has developed in the art for diodes that can be operated with lower forward voltage drops.